The invention relates to a method for operating an internal combustion engine having intake and exhaust valves the opening of the intake valve being adjustable depending on the engine operating conditions.
U.S. Pat. No. 6,688,280 B2 discloses a method for the reduction of nitrogen oxides which are generated during fired operation of an internal combustion engine. Here, full closing of the inlet valves of the cylinders is delayed such that, at the beginning of the compression stroke, the inlet valve remains in the open position and a part of the combustion air which has already been introduced into the combustion chamber can flow back again via the open inlet valve into the upstream air collector. In this way, both the combustion chamber internal pressure and also the combustion chamber temperature are lowered, which reduces the generation of nitrogen oxides.
The nitrogen oxide emissions can also be reduced by means of exhaust gas recirculation. Such a method is described for example in DE 197 34 494 C1, wherein during part-load operation of the internal combustion engine, a part of the exhaust gas mass flow is conducted out of the exhaust system and returned, via a recirculation line into the intake tract, where the exhaust gas is mixed with the combustion air which is taken in. With such an exhaust gas recirculation, however, there is the problem that, with an increasing recirculation rate, the risk of contamination or coking in particular of an exhaust gas cooler which is situated in the recirculation line is increased, whereby the thermal efficiency of the internal combustion engine, the power capacity of the engine and the exhaust gas properties are adversely affected.
It is the object of the present invention to lower the exhaust gas emissions using simple measures which do not adversely affect the service life of the internal combustion engine or of components of the internal combustion engine.